Vertical decomposing device for separating mercury and alkali metal hydroxides



- 19, 1970 1 I I H .HUND ETAL v 3,512,220v I VERTICAL DECOMPOSIQG nmvxcsFoa SEPARA'I'ING MERCURY AND ALKALI METAL HYDROXIDES Filed Sept. 19, 1967 NCIOH Hg/No INVENTORS HELMUT HUND FRANZ GLOS M, W 4 W ATTORNEYS United States Patent Olfice 3,512,220 Patented May 19, 1970 any Filed Sept. 19, 1967, Ser. No. 668,817 Claims priority, application 2Cgrmany, Sept. 23, 1966,

Int. Cl. Bbl 9/04 US. Cl. 23-288 3 Claims ABSTRACT OF THE DISCLOSURE A vertical decomposing device for separating mercury from alkali metal hydroxide, consisting of a zone of decomposition filled with graphite as catalyst and a washing zone improved by using in the washing zone intermediate plates in the form of alternating upright cones and suspended truncated cones.

The present invention relates to a vertical decomposing device for separating mercury contaminated with alkali metal from alkali metal hydroxide solution.

The alkali metal amalgam produced by electrolysis of alkali metal chloride in the so-called primary cell is decomposed in a decomposing device (secondary cell) by reaction with water in the presence of a catalyst, for example graphite, to yield alkali metal hydroxide solution and hydrogen. The mercury depleted of alkali metal, referred to in the following as mercury, is reconducted into the primary cell. There are known horizontal and vertical decomposition devices, the vertical devices having the advantage of being better adaptable to the efiiciency of the primary cell.

In a vertical decomposition device the amalgam is fed at the head in countercurrent flow with water and trickles over a packing of broken graphite pieces or graphite balls and the mercury is reconducted to the primary cell from the foot of the decomposing device.

The decomposition is the more complete the finer the amalgam in the reaction. The fine distribution of the amalgam is brought about by the shape and the size of the graphite pieces and the packing height thereof. The yery fine disintegration of the amalgam into droplets involves, however, the disadvantage'that at the foot of the decomposing device the mercury and the alkali metal hydroxide solution are difficult to separate from one another so that part of the produced alkali metal hydroxide is passed with the mercury from the decomposing device into the primary cell.

In the primary cell alkalinization of the alkali metal chloride brine leads to an undesired formation of hypochlorite or chlorate which does not only reduce the chlorine yield but also causes an increased corrosion of the anodes.

Because of the higher current densities in the primary cells now used it is necessary to construct compact, mercury-saving decomposing devices which ensure a satisfactory separation of the mercury from the alkali metal hydroxide.

To achieve this the residence time of the mercury in the bottom part of the decomposing device has been prolonged or an additional wash tank for the mercury has been installed between the cell and the decomposing device.

US. Pat. 2,588,469 proposes to contact mercury contaminated with alkali metal hydroxide solution, after it has left the decomposing device, with the water of decomposition in a special chamber or section. The chamber is closed towards the decomposing zone by an intermediate wall provided with non-return valves and water seals for the hydroxide solution, gas or vapour and mercury. An intermediate wall of this construction shall prevent the concentrated hydroxide solution from passing by diffusion from the zone of decomposition into the wash tank. The mercury enters the device through a water seal and flows downward over a series of alternately arranged plates, while the water of decomposition flows in countercurrent therewith through the non-return valve into the decomposing device. In spite of the technical equipment in the vertical decomposing device, an optimum separation of the mercury from the alkali metal hydroxide solution is not obtained.

The present invention provides a vertical decomposing device consisting of a decomposing zone filled with graphite as catalyst, a washing zone equipped with intermediate plates in which the mercury contaminated with alkali metal hydroxide solution and possibly alkali metal is washed in countercurrent flow with water of decomposition and a sieve bottom separating the washing zone from the zone of decomposition through which the mercury flows off uniformly over the entire cross-section of the decomposing device, the intermediate plates in the washing zone having the shape of alternating upright cones and suspended truncated cones.

The intermediate plates are suitably inclined by 1.5 to 2.5 mm. for 'mm. and the distance at the points of deflection is adjusted in such a manner that in the washing zone a concentration of the alkali metal hydroxide solution of at rnost 5% is maintained. The upright cones are provided with radial guide ribs.

The device according to the invention and the way how to use it are illustrated by way of example with reference to the accompanying drawing.

In the drawing, FIG. 1 represents a diagrammatical cross-section of the decomposition device according to the invention and FIG. 2 is a top view of an especially favourable shape of the upright cones.

The amalgam flows into the decomposing device through conduit 1 and while being decomposed it descends through the graphite packing 17 to sieve bottom 16. The mercury passes through the sieve bottom and flows in the direction of the arrows over the 4 intermediate plates 2 to 5, of which 3 and 5 are upright cones while 2 and 4 are suspended truncated cones, and leaves the device through conduit 6. The water of decomposition enters the device by conduit 7 and flows in countercurrent with the mercury. The alkali metal hydroxide solution is withdrawn through conduit 8 and the hydrogen through conduit 9. In order to ensure a uniform diffusion of the mercury film in the washing zone guide ribs 14 (FIG. 2) are mounted on intermediate bottoms 3 and 5, for example in vertical direction with respect to the axis of the decomposing device. On intermediate plate 3 stands a vent pipe 10 in axial position. Gaps 11 to 13 through which flows the mercury contaminated with alkali metal hydroxide solution in countercurrent with the water of decomposition are formed by the distances of the intermediate plates.

The following example serves to illustrate the invention but it is not intended to limit it thereto.

EXAMPLE With a current load of the primary cell of to kilo-amperes 50 tons per hour of mercury were circulated. The decomposition device had a height of 120 cm. and a diameter of 80 cm. At the points of deflection 15 the intermediate plates had a distance of 14 to 16- mm., they had an inclination of about 2 mm. for 100 mm. In the Washing zone the alkali metal hydroxide solution had a concentration of 0.1 to 5%. The hydroxide solution withdrawn at' the head of the column had a concentration of 40 to 50% and the mercury flowing oif at the bottom of the decomposing device contained less than 0.5% of alkali metal hydroxide solution, calculated on the amount of solution withdrawn at the head.

iln order to obtain the same result with a conventional decomposing device by increasing the residence time of the mercury in the device 35 to 50% more mercury is required. When a wash tank is used it is also necessary to use 20 to 30% more mercury and dilute sodium hydroxide solution is obtained in an amount of up to 5% of the production.

The vertical decomposing device according to the invention offers the advantage that the washing zone only requires a small space and that with a minimum amount of mercury required the alkali metal hydroxide is almost fully washed out of the mercury.

What is claimed is:

1. In a vertical decomposing device for recovering mercury from alkali metal amalgam, consisting of a decomposition zone filled with graphite as catalyst, a washing zone provided with intermediate plates in which the mercury contaminated with alkali metal hydroxide solution and alkali metal is conducted in countercurrent flow with the water of decomposition, and a sieve bottom to separate the decomposition zone from the washing zone, through which the mercury flows uniformly over the entire crosssection into the washing zone, the improvement which comprises using as intermediate plates in the washing zone alternating'upright cones and suspended truncated cones.

2. A device as claimed in claim 1, wherein means for inclining the intermediate plates 1.5 to 2.5 mm. per 100 mm. are provided for and the distance at the points of deflection is responsive to a concentration of the alkali metal hydroxide solution of at most 5% in the washing zone, said alkali metal hydroxide solution being based on the total amount of hydroxide solution in the decomposing device.

3. A device as claimed in claim 1, wherein on the intermediate plates having the shape of upright cones radial guide ribs are arranged in symmetric position with respect to the axis of the device.

References Cited UNITED STATES PATENTS 3/1952 Basilewsky 23l84 7/1957 Conforto 23-283 XR 

